Electrical measuring instrument.



'PATENTED MAY 22, 1906.

w, *5 mum. ELECTRICAL MEA$URING INSTRUMENT;

ATPLIOATIOH FILBD OGT.12, 1904.

UNITED STATES PATENT OFFICE.

WESLEY E. LAIRD, OF PITTSFIELD, MASSACHUSETTS, ASSIGXOR TO STANLEYELECTRIC MANUFACTURING COMPANY, OF PITTSFIELD,

.MASS ACHUSETTS, A CORPORATION OF NEW JERSEY. I

ELECTRICAL MEASURING INSTRUMENT.

Specification of Letters Patent.

Patented May 22, 1906.

Application filed October 12, 1904. Serial No 228,140.

To all whom it may concern:

' it known thatI, WEsLEY E. LAIRD, a citizen of the United States,residing at Pittsfield, in the county of Berkshire and State ofMassachusetts, have invented certain new and useful Improvements inElectrical MeasurmgInstrument-s, of which the following is aspeclfication.

My invention relates to electrical measuring instruments of thewell-known hot-wire type, and is particularly applicable to the form ofinstrument disclosed in Patent No. 537 ,343 issued to-Hartmann and BraunApril 9, 1895, though it is not limited thereto.

In measuring instruments which depend upon the expansion of a wiretraversed by an electric current for measuring the amount of the currentit is well understood that some provision must be made for guardingagainst variations in the indications given by the instrument duetovariations in atmospheric temperature. One method of guarding againstthis source of error that has been suggested heretoforeconsist-s'in'attaching the hot wire to a yielding abutment, such as aspring, which is supported in place by a compensating wire of'the samematerial as the ot Wire, so that variations in the tempera ture of thesurrounding air produce p'roportional variations in the length of thehot wire and compensating wire and the movement of the yielding abutmentcompensates for the expansion of the hot wire. In order that thiscompensation should be exact, it is nec iessary that the movement of theyielding abutment through a given distance should correspond to equal orproportional expansions of the hot wire and compensating wire. Thus ifthe two wires are of approximately e ual length and approximatelyparallel to e ch-other the abutment should move in a rli rectionsubstantially parallel to both wires."

' One feature of my invention consists in providing a novel and compactform of yielding abutment for this purpose which has a movement undervariation of atmospheric .tdmperature more nearly parallel to the wiresthan can be obtained with the single leafspring frequently employed.

In many instruments of the hot-wire type as heretofore built a sourceoferror has existed in the influence of the hot wire upon the preferablyof heat-absorbing and heat-conducting material, such as metal, so that.the heat from the hot wire is absorbed and radi ated into thesurrounding air or conducted through the supports of the heat-guard tothe casing.

In measuring instruments of all types it is sometimes desirableto-obtain a variation in the length of different parts of the scalecorresponding to the electrical units to be measured. Sometimes it isdesirable to obtain a perfectly even scale, so that a given movement ofthe needle corresponds to a given variation in the electrical quantitiesto be measured, regardless of the position of the needle on the scale.In other cases it is desirable that the reading over a portionof therange of the instrument should be very accurate, while the rest of therange is comparatively unimportant. In such a case the desired resultmay be obtained by relatively increasing the movement of the needle fora given variation in the current to be measured at that part of theinstruments range where the greatest accuracy of reading is desired.

One feature of my invention consists in providing instruments, in whichthe expansion of the hot wire is transmitted to the indicating means bya cord or thread which passes around a pulley, with a pulley not round,as is the usual construction, but shaped to give any desired an ularmovement of the ulley for a given longitudinal movement 0 the threadpassing around it. By this means the length of the units of the severalportions of the scale may be varied as desired. 7

My invention will best be understood by reference to the accompanyingdrawings, in which IOO Figure -1 shows a front elevation of 'aninstrument' arranged in accordance with my in-' vention. Fig. 2 shows aside elevation,

partly in cross-section, of the same; and Fig.

3 shows a detail of the pulley.

' In the drawings, A represents the back of 6 carried at their upperends, is also attached the co'mpensating wire E, the other end of WhlCllis secured to the posts. Thisc'onstruction with two parallel springsconnected *at both' ends gives a movement through the 7 range oftemperature 'to which the 'instrument is likely to be subjected whichmuch more closely approximates a. straight line parallel to the wiresthemselves than could be obtained by the use of a single spring of thesamelength, since with .a single spring the bending occurs chiefly nearthe free end, which causes the end to move substantially in the arc of acircle having a radius much less than the len 'th of the spring, whilewith two sprin s rigid connected at both ends a compound bend occursnear their middle, which is equivalent to a movement on an arc ofgreater radius- I In order to prevent radiation h 'om' the hot wire ,tothe compensating wire, the heatguard F is interposed between the two.This heat-guard, which, as has been said before, is preferably ofheat-absorbing and heat-conducting material, eil'ectually screens thecompensating wire from the hot wire and prevents errors in the readingsdue to the influence of the hot wire upon the compensating wire.

The expansion of the hot wire B is transmitted to the indicating-needleN by the cord G, one end of which is attached to a oint on the hot wireand the other end of whic is supported by the post 9, and by the-cord h,which is attached to the cord G, passes round-apullev I on the shaft Jwhich carries the needle.

X, and is held taut by the tension-spring K, mounted on the post k. 1The shaft J is su ported in bearings 0, carried bythe post 8, andthis'shaft is provided with a conductingsector-L, which moves betweenthe poles of a permanent damping-magnet M. 'ith the arrangement asdescribed it will be seen that a very sllght expansion of the wn'e Bwill result 1n a very considerable longitudinal movement of the cord H,and consequently a considerable angular movement of the needle. X

over the scale. S. In order to obtain the desired proportion between theunitsin the sevsaopva eral which is eccentr cally mounted or .is cam-.shaped, asshown in Fi 3. By properly shaping this pulley, as in icated,the relative proportion o the units on the-different parts of the scalemay be adjusted as desired. Since the sector Lacts as a counterbalancefor the needle N, it ma be given a comparatively large radius wit outunnecessarily increasing the weight of the moving parts, andconsequently efficient damping is obtained with a com aratively weakmagnetic field, so that. a fair y wide air gap may be employed-- in thepermanent magnet, wlnch simplifies the construction of the instrument.

I do not desire toclimit myself to the particular construction andarrangement oi parts here shown, but aim in the appended claims to coverall modifications w within the scope of my invention.

What I claim as new, and desire tosecure by Letters Patent of the UnitedStates, is-

1. In an electrical measuring instrument, a yielding abutment comprisingtwo parallel springs supported at one end and connected to each 0t er attheir free end, a wire adapted to be traversedghy electric currentstretched between saidabutment and another support, and-a compensatingwire approximately arallel to the first wire connecting said a utment toa fixed sup ort.

2Q Inan electrica measuring instrument, 9.

arts of the scale, I provide a. pulley ch are.

yielding abutment comprising two parallel leafsprings.'supported.at oneend and connected to each other at their free end, a wire adapted to betraversed by electric current stretched between said abutment andanother support, and a compensating wire approximately parallel to thefirst wire and ta ing u the tension of said parallel springs.

3. n an electrical measuring instrument, a yielding abutment comprisingtwo parallel springs su ported at one end and connected to each ot ierat their free end, a'wire adapted to be traversed by electric currentstretched at substantially right angles to said springs and attached totheir free end, and a compensating wire substantially ara'llel to saidfirst wire and connecting the ee end of said springs to afixed support.

4:. In an electrical measuring instrument, a wire adapted to betraversed by electric currents, a compensating wire not traversed bycurrent, and a heat-guard interposed between said wires.

5. In an electrical measuring instrument, a wire adapted to be traversedby electric cur-. rents, a compensating wire not traversed by current,and aplat-e of heat-absorbing, heat conducting, material interposedbetween said wires.

6. In an electrical measuring instrument,

an expansion-wire adapted to he traversed lioby electric .currents,means for laterally straining said wire, means for indicating the.

amount said Wire yields to said strain when expanded by an electriccurrent, a compensating Wire adapted to vary the distance between theend supports of the eX a'nsionwire, and a heat-guard interposed etweensaid Wires.

7. In an electrical measuring instrument, indicating means, a cam-shapedpulley operatively connected thereto, a member movable With variation ofcurrent through the instrument, and a cord connected to said member andpassing around said pulley.

8. In an electrical measuring instrument, an expansion-Wire adapted tobe traversed by electric current, indicating means, a camshaped pulleyoperatively connected to said means, and a cord operativ y connected tosaid Wire and passing around said pulley.

9. In an electrical measuring instrument, an expansion-wire adapted tobe traversed by electric current, means for laterally strainingsaidwire, indicating means, a cam-shaped pulley carried thereby, and acord connected to said straining means and passing around said pulley.v

In witness vi /hereof I have hereunto set my hand this 8th day ofOctober, 1904.

, WESLEY E. LAIRD. Witnesses:

, SAMUEL E. WII DIFIELD,

R. E. HAYNES.

